Systems and methods for implementing receiver transparent Q-mode

ABSTRACT

In a receiver transparent Q-mode, i.e., a Q-mode that is only implemented by a transmitter, the receiver is unaware of the Q-mode state of the transmitter. In this type of Q-mode configuration, the transmitter could enter and exit Q-mode as desired while the receiver, could, for example, continue to function as if operating normally, such as in “showtime.” Through this approach, it is not necessary for the receiver to detect the transmitter&#39;s entry and exit of Q-mode.

RELATED APPLICATION DATA

[0001] This application claims a benefit of and priority to U.S.Provisional Application Serial No. 60/278,936 filed Mar. 27, 2001,entitled “Receiver Transparent Q-Mode,” U.S. Provisional ApplicationSerial No. 60/283,467 filed Apr. 12, 2001, entitled “ReceiverTransparent Q-Mode With On-Line Reconfiguration,” U.S. ProvisionalApplication Serial No. 60/287,968 filed May 1, 2001, entitled “ReceiverTransparent Q-Mode With On-Line Reconfiguration And Scrambling,” andU.S. Ser. No. 60/293,034 filed May 23, 2001, entitled “ReceiverTransparent Q-Mode With On-Line Reconfiguration And Scrambling AndQ-Mode Symbol Distortion,” all of which are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention general relates to multicarrier communicationsystems. In particular, this invention relates to systems and methodsthat allow a transmitter to enter and exit a Q-Mode.

[0004] 2. Description of Related Art

[0005] Q-mode is being examined by ADSL standards bodies in the ITU-T inthe development of the G.dmt.bis and G.lite.bis ADSL standards, both ofwhich are incorporated herein by reference in their entirety. Q-mode isa low power transmission mode intended to save power by transmittingsignals with lower PAR (Peak to Average power Ratio) with respect tonormal steady state, i.e., full power, signals. The Q-mode signal withlow PAR will often have the same average power as the normal steadystate signals but since the peak power is reduced, power consumption canbe reduced in the analog transmission circuitry. This is very importantespecially for saving power in telephone company central offices andremote cabinets where ADSL modems are often installed.

[0006] Current Q-mode proposals utilize a Q-mode “filler” symbol withlow PAR properties in order to save power at the transmitter. Discussionof this type of approach can be found in various ITU Documents, such as,BA-044, BA-045, HC-029R1, CF-033 and CF-040, all of which areincorporated herein by reference in their entirety. Other ITU proposalsstate that the filler symbol should be defined by the transmitter andcommunicated to the receiver during initialization, see BI-080 and D.87,both of which are also incorporated herein by reference in theirentirety.

SUMMARY OF THE INVENTION

[0007] However, current systems suffer from a number of drawbacks. Theseexemplary drawbacks include the necessity of receivers being required toimplement circuitry to detect Q-mode symbols on every DMT symbol.Furthermore, the entry and exit modes associated with the Q-mode lackrobustness since the receiver needs to be able to distinguish a Q-modesymbol from a real information-carrying symbol.

[0008] In contrast, the exemplary systems and methods of this inventionfocus on a receiver transparent Q-mode, i.e., a Q-mode that is onlyimplemented by the transmitter, wherein the receiver is unaware of theQ-mode state of the transmitter. In this type of Q-mode configuration,the transmitter could enter and exit Q-mode as desired while thereceiver, could, for example, continue to function as if operatingnormally, such as in “showtime.” Through this approach, it is notnecessary for the receiver to detect the entry and exit of Q-mode by thetransmitter.

[0009] Accordingly, exemplary aspects of the present invention relate tomulticarrier communications systems. In particular, an exemplary aspectof the invention relates to conserving power at a transmitter.

[0010] Additionally, aspect of the present invention relate to allowinga transmitter to enter into a Q-mode while the receiver is unaware ofthe operational state of the transmitter.

[0011] Aspects of the present invention also relate to eliminating theneed for a receiver to have Q-mode entry and exit detectioncapabilities.

[0012] Aspects of the present invention also relate to seamless changinginto and out of a Q-mode between a transmitter and a receiver.

[0013] Additional aspects of the present invention relate to sending aplurality of symbols from a transmitter to a receiver wherein thereceiver does not need to determine which of the symbols is a Q-modesymbol.

[0014] These and other features and advantages of this invention aredescribed in, or are apparent from, the following detailed descriptionof the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The embodiments of the invention will be described in detailed,with reference to the following figures, wherein:

[0016]FIG. 1 is a block diagram illustrating an exemplary portion of atransmitter according to this invention;

[0017]FIG. 2 illustrates an exemplary constellation point according tothis invention;

[0018]FIG. 3 illustrates an exemplary 64-QAM constellation according tothis invention;

[0019]FIG. 4 is block diagram illustrating an exemplary XOR scrambleraccording to this invention;

[0020]FIG. 5 is block diagram illustrating an exemplary transmitterhaving an XOR scrambler and phase rotator for a 64-QAM constellation;

[0021]FIG. 6 is a block diagram illustrating an exemplary transmitterhaving an XOR scrambler and phase rotator for use with trellis coding;

[0022]FIG. 7 is flowchart illustrating an exemplary method of trainingthe transmitter and receiver according to this invention;

[0023]FIG. 8 is flowchart illustrating an exemplary method of enteringQ-mode according to this invention; and

[0024]FIG. 9 is flowchart illustrating an exemplary method of exitingQ-mode according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIG. 1 illustrates a exemplary transmitter according to anembodiment of this invention. In particular, the transmitter 10comprises a serial to parallel converter 110, an XOR scrambler 120, aQAM (quadrature amplitude modulation) encoder 130, a phase rotator 140,an Inverse Fast Fourier Transform module 150, a sign inversion module160 and a pseudo-random bit sequence (PRBS) module 170, allinterconnected by links 5.

[0026] The exemplary systems and methods of this invention will bedescribed in relation to a multicarrier modulation communication system.However, to avoid unnecessarily obscuring the present invention, thefollowing description omits well-known structures and devices that maybe shown in block diagram form or otherwise summarized. For the purposesof explanation, numerous specific details are set forth in order toprovide a thorough understanding of the present invention. It should beappreciated however that the present invention may be practiced in avariety of ways beyond these specific details.

[0027] While the exemplary embodiments illustrated herein show thevarious components of the transmitter, and corresponding receiver,collocated, it is to be appreciated that the various components of thesystem can be located a distant portions of a distributed network, suchas a telecommunications network and/or the Internet or within adedicated communications network. Thus, it should be appreciated thatthe components of the transmitter and receiver, respectively, can becombined into one or more devices or collocated on a particular note ofa distributed network, such as a telecommunications network. As will beappreciated from the following description, and for reasons ofcomputational efficiency, the components of the communications networkcan be arranged at any location within the distributed network withoutaffecting the operation of the system.

[0028] Furthermore, it should be appreciated that the various linksconnecting the elements can be wired or wireless links, or a combinationthereof, or any known or later developed element(s) that is capable ofsupplying and/or communicating data to and from the connected elements.

[0029] As used herein, the following nomenclature is represented by thefollowing symbols wherein:

[0030] N total number of carriers used for transmission

[0031] C_(i) the i^(th) carrier, where I=0 . . . N

[0032] B_(i) the number of bits modulated by C_(i) (as defined by thereceiver during training)

[0033] G_(i) the fine gain of C_(i) (as defined by the receiver duringtraining)

[0034] Q_(i) the 4-QAM constellation point of C_(i) of the Q-mode symbol(as defined by the transmitter)

[0035] w_(i) the input bit word to be modulated on C_(i) (with thelength of this word being B_(i) bits)

[0036] s_(i) the XOR scrambled w_(i)

[0037] q_(i) the QAM symbol of C_(i)

[0038] X_(i) the XOR scrambler bit values for C_(i) and

[0039] R_(i) the phase rotation value of C_(i)

[0040] In operation, L bits are received from a transmission protocolspecific-transmission convergence layer (not shown) and converted, withthe cooperation of the serial to parallel converter 110, into w_(N)words. These W_(N) words are then processed by the XOR scrambler 120 andphase rotator 130, which work in cooperation, to map and “all zeros”input word (w_(i)) of the i^(th) carrier to a point in a constellationdefined by B_(i),G_(i) that is closest to the 4-QAM Q-mode constellationpoint (Q_(I)) for that carrier. At the receiver, not illustrated, theinverse operations are performed.

[0041] As discussed above, an exemplary purpose of the XOR scrambler 120and the phase rotator 140 is to map the “all zeros” input word (w_(i))to the constellation point that is closes to the Q-mode constellationpoint (Q_(i)) for each carrier. As a result, a different PMD scramblerand phase rotation is defined for each carrier. The XOR scrambler 120and phase rotator 140 depend on, for example, the QAM constellation size(B_(i)) of the carrier, the fine gain (G_(i)) of the carrier and theQ-mode 4-QAM constellation point (Q_(i)) of the carrier which can be,for example, defined by the transmitter or receiver.

[0042] As illustrated in FIG. 1, the XOR scrambler 120 and phase rotator140 operate on a carrier by carrier bases. Thus, as illustrated inexemplary FIG. 2, and assuming that the carrier has been specified bythe transmitter to utilize Q_(i)={+,−} or {0, 1} for the Q-mode symbol,the XOR scrambler bits (X_(i)) and the phase rotation value (R_(i)) of acarrier that has B_(i)=6 bits and a fine gain of G_(i)={squareroot}(21/13)=1.27 (2.08 db). After scaling by the factor G, FIG. 3illustrates the 64-QAM constellation, wherein G=G_(i)*(constellationgain)={square root}(21/13)(1/{square root}(21))={square root}(1/13),wherein 1/{square root}(21) is the constellation gain scaling for a64-QAM constellation relative to the 4-QAM constellation. Based on thisscaled 64-QAM constellation, the constellation point in the lower righthand quadrant that is closest to the {+,−} constellation point of theQ-mode signal in FIG. 1 can be determined. A pure phase rotation of theentire constellation allows the use of any 64-QAM constellation point,i.e., not only those on the diagonal. Therefore, the 64-QAMconstellation point that has a distance of {square root}(2) from theorigin need to be determined since this is the distance of theQ_(i)={+,−} Q-mode point. The distance from the origin of the 64-QAMconstellation points in the lower right hand quadrant are:

D(1,1)=G*{square root}(1+1)={square root}(1/13)*{square root}(2)

D(1,3)=D(3,1)=G*{square root}(1+9)={square root}(1/13)*{square root}(10)

D(1,5)=D(5,1)+G*{square root}(26)={square root}(1/13)*{squareroot}(26)={square root}(2)

D(3,3)=G*{square root}(9+9)

D(3,5)+D(5,3)=G*{square root}(9+25)

D(5,5)=G*{square root}(25+25)

D(1,7)=D(7,1)=G*{square root}(1+49)

D(3,7)=D(7,3)=G*{square root}(9+49)

D(5,7)=D(7,5)=G*{square root}(25+49)

D(7,7)=G*{square root}(49+49)

[0043] From the calculation it is apparent that the point at (1,5) has adistance of {square root}(2) from the origin and is chosen as theconstellation point to be used for the “all zeros” input word on thisexemplary constellation. The (1,−5) constellation point is mapped to thebit pattern {010101 } in the 64-QAM constellation. Therefore, thescrambler for this constellation point will map the all-zeros word{000000} to {010101}. A simple scrambler that can accomplish this is anXOR of the input word for this constellation with the bit patternX_(i)={010101}.

[0044]FIG. 4 illustrates an exemplary XOR scrambler that is capable ofperforming the scrambling set forth in the above example. Specifically,the XOR scrambler receives an input word to be modulated that comprisesall zeros, XORs that with the XOR scrambler bit values for C_(i) andoutputs the appropriately scrambled XOR scrambled input word s_(i). Asan example, FIG. 4 shows the XOR scrambler for a carrier using theQ-mode constellation shown in FIG. 2 and the 64-QAM constellation shownin FIG. 3 using the fine gain of 2.08 dB.

[0045] In addition to the XOR scrambler illustrated in FIG. 4, the phaserotation value R_(i) needs to be specified in order to migrate the(1,−5) constellation point to the diagonal. A phase rotation of R=π/5.34will place the (1,−5) point on the diagonal 64-QAM constellation. Ofcourse, this phase rotation will be applied to the entire 64-QAMconstellation thus maintaining the same distance properties and averagepower for the entirety of the constellation.

[0046] As illustrated in FIG. 5, the phase rotation is applied by thephase rotator 140 after the QAM encoding is performed by, for example,the 64-QAM encoder 130. The result is then passed through the IFFTmodule 150 and output.

[0047] As discussed above, each carrier will have a different XORscrambler and phase rotation. The parameters for the XOR scrambler 120and the phase rotation for the phase rotator 140 for each carrier dependon, for example, the QAM constellation size for the particular carrier,the fine gain value used on that carrier and the Q-mode 4-QAMconstellation point of that carrier.

[0048] Even though not illustrated, at the receiver, the inversefunctions to that of the transmitter are performed. Specifically, thereceived signal is rotated back at the output of an FFT module anddescrambled at the output of the QAM decoder to obtain the originalinput word w_(i).

[0049]FIG. 6 illustrates an example of XOR scrambling and phase rotationusing trellis coding. In particular, where trellis coding is used, theXOR scrambler 120 scrambles only the uncoded bits. The coded bits willpass through the convolution encoder 125 unscrambled. This results in anall-zeros word always being mapped to a constellation point in coset 0.If the constellation point used to map the all-zeros input word to the4-QAM Q-mode constellation pint is not in coset 0, then the entireconstellation can be rotated to produce the correct results.

[0050] As an illustrative example, assume trellis coding is used for theabove example. In the above example, the chosen constellation point(1,−5) is located in coset 1. However, in the (+,+) quadrant theconstellation point (5,1) is located in coset 0. Therefore, rotating theentire constellation by −π/2 will move the (5,1) constellation point tothe (1,−5) position and, as a result, the rotated (1,−5) constellationpoint will be located in coset 0.

[0051]FIG. 6 illustrates this encoding process. In particular, the bitvalues mapped to the unrotated (5,1) constellation point are {010000}.In this case, the XOR scrambler value is X_(i)={0100} is applied by theXOR scrambler 120 to only to the uncoded bits (4 MSBs). The coded bits(2 LSBs) that designate the coset will be encoded with the convolutionencoder 125 without scrambling. The resulting QAM symbol is located incoset 0 at constellation point (5,1). A phase rotation of −π/2 is thusrequired to move this QAM symbol to the (1,−5) position. Thus the −π/2phase rotation is added to the phase rotation R_(i)=π/5.34−π/2 by thephase rotator 140.

[0052] Alternatively, for example, the XOR mapping is applied to allbits, even when trellis coding is enabled. In this case, however, anadditional step is required in the receiver's trellis decoder. Normally,the first step in trellis decoding is to determine the four 2-D metricsfor each pair of constellation points. These 2-D metrics are thencombined to form eight 4-D metrics. If the coset bits are XORed at thetransmitter, then this XORing be accounted for in the determination ofthe 4-D metrics prior to the normal decoding. A simple way to doaccomplish this is to modify the 4-D metric computation table with theappropriate XOR values.

[0053] For example, let C₂ ^(i) be the i^(th) 2D coset and let C₄ ^(i)be the i^(th) 4D coset. By definition C₄⁰ = min {C₂⁰ + C₂⁰, C₂³ + C₂³}.

[0054] Now, let H₁ be the 2 bit XOR pattern for the first 2D tone, andlet H₂ be the 2 bit XOR pattern for the second 2D tone. The 4D metriccomputation is thus:C₄⁰ = min {C₂^(0   ⊕ H₁) + C₂^(0   ⊕ H₂), C₂^(3   ⊕ H₁) + C₂^(3   ⊕ H₂)}

[0055] Therefore, the XOR operation can be removed at the receiverduring the determination of the 4D metrics. Note that the 2 bit patternis simply the first 2 bits of the XOR pattern implemented at thetransmitter, and the higher level XOR bits can be removed after thetrellis decoding.

[0056] The transmitter and receiver can also determine a loss in margin,if any, when the transmitter sends the Q-mode symbol based on adifference between the Q-mode symbol and the scrambled all-zeros symbol.This loss and margin occurs, for example, if the receiver does not mapthe all-zero symbol exactly to the Q-mode symbol. While it can beacceptable to have a loss in margin during transmission of the Q-modesymbol since the Q-mode symbol contains no information bits, in the caseof a large margin loss, the transmitter and receiver must account for,for example, possible false alarms such as CRC errors, FEC errors, HECerrors, and the like, during the Q-mode symbol transmission. Otherwisedata transmission is unaffected.

[0057] As discussed above, a loss in margin occurs, for example, if thereceiver does not map the all-zero symbol exactly to the Q-mode symbol.Alternatively, a loss in margin can be reduced by using the fine gains.In particular, the receiver transparent Q-mode places restrictions onthe values of the fine gains that may be used if it is desired to have areceiver without any loss in margin during the reception of the Q-modesymbol. For large constellations, such as, for example, B_(i) 6 bits,there are several possible fine gain values that will align a point inthe constellation with the Q-mode constellation point. For example, forsmaller constellations, such as B_(i) 6, there are fewer allowable finegain values.

[0058] A number of exemplary issues can determine the number of finegain values. In particular, the fine gain can be chosen such that eitherthe exact Q-mode constellation point is used, or the fine gain is suchthat the actual Q-mode constellation point can be transmitted without aloss of margin even though the transmitted point is not the point thatwould have be chosen during a showtime transmission. In other words, incertain cases, the constellation point can be forced to the value thatis needed for Q-mode without a loss of margin at the detector. Thisoccurs when an all-zero input is mapped to the outermost constellationpoint along one of the 45° diagonals and the Q-mode symbol chosen to liefurther along this diagonal.

[0059] Table 1 illustrates exemplary allowable fine-gain values (in dB)if a receiver operates with zero margin loss during Q-mode transmission.TABLE 1 B_(i) = B_(i) = B_(i) = B_(i) = B_(i) = B_(i) = B_(i) = B_(i) =1 2 3 4 5 6 7 8 <0 <0 −2.2 <−2.55 −2.3 <−3.7 −3.1 <−4.2 0 −1.1 −2.4 −2.5−3.6 0.45 −1.4 −2.0 −3.1 3.0 −0.75 −1.7 −2.0 0.92 −1.1 −2.6 2.08 −0.40−2.4 0 −2.3 0.44 −2.1 1.5 −1.7 2.1 −1.7 3.8 −1.5 −1.4 −1.3 −1.2 −1.1−0.75 −0.57 −0.20 0 0.21 0.66 1.2 1.4 2.0 2.4 2.8

[0060] With reference to Table 1, note that the exemplary fine gainvalues are valid for a Q-mode symbol with no fine gain, i.e., g_(sync)=0dB.

[0061] For B_(i)=1 and B_(i)=2, the receiver may use any negative finegain value since the Q-mode constellation point can be mapped to a pointon the diagonal and therefore the Q-mode symbol will not effect the biterror rate (BER) performance and, in actuality, will increase theminimum distance properties of the constellation during the Q-modetransmission.

[0062] The same principles can be applied to all even constellations,i.e., the negative fine gains that result in the Q-mode constellationpoint being mapped to a point that is further from the origin than theoutermost constellation point of the B_(i) constellation do not resultin a loss in margin during transmission of the Q-mode symbol. However,

[0063] Exemplary advantages of the above-described receiver-transparentQ-mode include design flexibility for a receiver. In particular, thereare a plurality of exemplary trade-offs that a receiver my make inimplementing the exemplary systems and methods of this invention.

[0064] First, there is a data rate trade-off where a receiver may choseto use only the fine gain values in Table 1 if the receiver wants toassure loss in margin during reception of the Q-mode symbol. In doingso, there may be a small loss in data rate. However, this data rate lossmay be an acceptable trade-off for a receiver that does not implementthe complexities of detecting the entry and exit Q-mode symbols. Forexample, bit loading algorithms may be used to avoid a loss in datarate.

[0065] Alternatively, a receiver may chose not to use the fine gainvalues in Table 1 and still operate in a receiver-transparent Q-mode. Asa result, there can be an effective decrease in margin during thereception of the Q-mode symbol. However, this decrease in margin duringthe Q-mode symbol will not effect the true information carrying symbolsand therefore may be an acceptable trade-off for a receiver that doesnot wish to implement the complexities of detecting the Q-mode entry andexit symbols.

[0066] Furthermore, a receiver may chose to not use the fine gain valuesin Table 1 and not operate in a receiver-transparent Q-mode. In thiscase the receiver may chose to operate as in the current Q-modeproposals in G.dmt.bis and implement the complexities of detecting theentry and exit Q-mode functions. The receiver could notify thetransmitter that it is not using the XOR and phase rotation functionsand this could be accomplished by sending, for example, a message fromthe receiver to the transmitter indicating this type of operation.Alternatively, the receiver could send to the transmitter an X_(i) tablewith all-zeros, such that the all-zeros input words are mapped to thetrue all-zeros constellation point.

[0067] Thus, one exemplary benefit of the systems and methods of thisinvention is that it allows the receiver to decide what method thereceiver will be using for Q-mode. Therefore, the complexity burden toimplement the Q-mode can be, for example, determined by the transmitterwhich, if the transmitter opts to implement Q-mode, the transmitter willimplement the XOR scrambler and phase rotation even if the receiverdecides to not use those functions, i.e., the receiver chooses to detectthe Q-mode filler signal.

[0068] In the current Q-mode proposals in G.dmt, the Q-mode signal israndomized by alternating the reverb/segue signals based on a PRBS withperiods longer than 4,000 symbols. The same method of randomization canbe used in conjunction with the systems and methods of this invention.However, in this case, all DMT symbols, both information-carrying DMTsymbols and the Q-mode symbol, are inverted based on the PRBS. This canbe implemented by, for example, alternating the signs of all DMT symbolsat the IFFT output based on the PRBS. Alternatively, the phase shift maybe implemented as part of the phase rotation in the phase rotator 140.This will allow, for example, phase shifts other than just 180°, i.e.,simple inversion. For example, the DMT symbols could be furtherrandomized based on the 90° phase shifts.

[0069] Multicarrier ADSL systems typically use a synchronous bitscrambler before modulation in order to assure that the data bits beingtransmitted are as random as possible, this is important to keep thePeak to Average Power ration low in multicarrier modems. The XORscrambler 120 as discussed herein however, does not provide thisrandomization function since it is primarily just mapping bits from onepattern to another. In order to support the exemplary bit scramblingfunctions according to this invention, one of the following exemplaryembodiments can be implemented:

[0070] First, a self-synchronizing scrambler can be placed before theXOR scrambler at the transmitter. If FEC coding is used, then theself-synchronizing scrambler would typically be placed before the FECcoder at the transmitter. The self-synchronizing scrambler is then resetafter scrambling S bits. Upon the scrambler being reset, the initialscrambler state is reset to all zeros. Also, upon the scrambler reset,the scrambler feedback connections may be changed. For example, thescrambler feedback may be changed based on a different pseudo-randompattern. For example assume the scrambler was defined as:

D _(n) ′=D _(n) ⊕D _(n−17) ′⊕D _(n−25)′,

[0071] where D_(n) is the n^(th) data bit at the input of the scramblerand D_(n)′ is the n^(th) data bit at the scrambler output. After S bitsare input to the scrambler, the scrambler is reset to the all zero stateand the feedback connections changed to, for example, n−3 and n−20.These new connections can be updated, for example, based upon a secondscrambler, and the number of connections need not be fixed. The value ofS in the second scrambler, or for example a random number generator,that defines the new delay values would be known by the transmitter andthe receiver so that the receiver could reverse the scramblingoperation, i.e., descramble, upon scrambler reset.

[0072] Another method which does not involve changing the scramblerconnections is based on defining S in such a way as to assure that arepetitive bit pattern at the input of the scrambler will not result inthe same bit pattern being modulated on the same carriers in a pluralityof DMT symbols. For example, this can be accomplished by defining S andL, where L is the number of bits modulated on all the carriers in singleDMT symbol, to have no common factors other than 1, i.e., S and L aremutually co-prime.

[0073] Second, the data bits can be interleaved (shuffled) at some pointprior to the XOR scrambler. This interleaving pattern can change fromone DMT symbol to the next. The interleaving pattern could be known bythe transmitter and receiver so that the operations can be reversed bythe receiver. For example, assume that 1 is the number of bits modulatedon all carriers in a single DMT symbol. A simple interleaver could moveN bits to the end of the length L bit stream on the N^(th) DMT symbol.For example if [B1,B2,B3,B4 . . . BL] was the bit pattern at the inputof the interleaver on the 3rd DMT symbol, then [B4,B5,B6, . . .BL,B1,B2,B3] would be the bit pattern at the interleaver output.However, it should be appreciated that any other interleaving patterncan be used as long as it is known by the transmitter and the receiver.

[0074] As an alternative to the above exemplary embodiments of thesystems and methods of this invention, instead of having the receiverdefine the B_(i) and G_(i) tables such that the Q-mode symbol is mappedto the all-zero symbol, the Q-mode symbol may be defined by thetransmitter and be restricted to valid constellation points. Theresulting Q-mode symbol may not have the exact PAR characteristicsdesired by the transmitter, but this Q-mode symbol will also map exactlyto the all-zero symbol.

[0075] Constellation points chosen for the Q-mode symbol may bedetermined by the transmitter and communicated to the receiver after theB_(i) and G_(i) tables have been communicated. Alternatively, theconstellation points chosen for the Q-mode symbol may be determinedindependently by the receiver and the transmitter based on the desiredQ-mode symbol, such as a Q-mode symbol sent from the transmitter to thereceiver during training.

[0076] Alternatively, tone reordering is another method that can be useduring bit loading to achieve better mapping of the all-zero symbol tothe Q-mode symbol. This can be helpful when trellis coding or some othercode, such as turbo code, is used.

[0077] Alternatively still, the Q-mode signal may be also be predefinedin a standard, or, for example, by a system provider, as opposed to bedefined by the transmitter.

[0078] Furthermore, instead of exchanging the X_(i) table, thetransmitter and receiver may independently determine the X_(i) values byfinding the constellation points that are closest to the Q-modeconstellation point. From this value, the transmitter and receiver canindependently generate the XOR scrambler and the phase rotation valuesrequired. In order to determine the X_(i) values independently, thereshould be a set of rules defined to ensure that the same constellationpoints are chosen by both the transmitter and receiver. This isimportant when two or more constellation points satisfy the condition ofbeing closest in magnitude to the Q-mode point. For example, in thedisclosed example, the constellation points (1,−5) and (5,−1) are the{square root}2 distance from the origin. In this case, a rule such as“always pick the point that results in the smallest positive phaserotation value” can be chosen and this would ensure that both thetransmitter and receiver would be pick the constellation point (1,−5).Alternatively, tables can be stored at one or more of the transmitterand receiver and values chosen from those tables for X_(i) values.

[0079] In the case where a synchronized symbol is being transmittedevery 69 DMT symbols, as is done in the G.992.1 and G.992.2 standards, asystem with a receiver transparent Q-mode operating in accordance withthe systems and methods of this invention can send a Q-mode symbol as aSYNC symbol. This way, when the transmitter enters the Q-mode, the SYNCsymbol could continue to be sent unaltered.

[0080] Still further, on-line reconfiguration is possible such that whenperforming changes to the B_(i) and G_(i) tables during showtime, e.g.,bit swapping on-line reconfiguration, in addition to the new B_(i) andG_(i) values, the receiver could also send to the transmitter the newcorresponding X_(i) values. If the receiver and the transmitter areindependently determining the X_(i) values, then it is not necessary forthe receiver to send the new X_(i) values during a showtime change ofthe B_(i) and G_(i) tables.

[0081] The ADSL framing parameters can also be configured in such a wayas to ensure that non-scrambled data sequences will be mapped todifferent carriers from one DMT symbol to the next. As an example, ATMcell headers are not scrambled by the ATM TPS-TC. If the number of bitsin a DMT symbol is chosen to be mutually co-prime with respect to theMUX data frame size in the ADSL modems, then when the SYNC bits areinserted into the data bit stream, the ATM cell headers will be mappedto different carriers from one DMT symbol to the next. Alternatively, ifthe number of bits in a DMT symbol is chosen to be mutually co-prime,with respect to the ATM cell size, such as 53 bits, then the ATM cellheaders will be mapped to different carriers from one DMT symbol to thenext. Alternatively still, if the number of bits in a DMT symbol ischosen to be mutually co-prime with respect to the Reed-Solomon codeword size, e.g., 53 bits, then the ATM cell headers will be mapped todifferent carriers from one DMT symbol to the next. Obviously,configuring other framing parameters in this manner will have the sameresults.

[0082]FIG. 7 outlines an exemplary method of training a transmitter anda receiver according to this invention. In particular, control begins instep S100 and continues to S110. In step S110, the transmitter specifieson all carriers the Q-mode filler symbol as any pseudo-random 4-QAM DMTsymbol. During this training, the transmitter sends the bit pattern(Q_(i)) for the Q-mode symbol to the receiver before the B_(i) and G_(i)tables are determined. Next, in step S120, the receiver determines theB_(i) and G_(i) tables. Alternatively, the receiver may take intoaccount the Q-mode symbol characteristics during bit loading in order toensure that a scrambled all-zero symbol is as similar as possible to thetransmitter defined Q-mode symbol. As described above, this can beaccomplished by mapping the all-zeros input word to a constellationpoint that is chosen to be as close as possible, or even identical, tothe Q-mode constellation point for the carrier. Thus, in step S130, adetermination is made whether the receiver will take into account theQ-mode symbol characteristics during bit loading. If the receiver does,control continues to step S140 where and all-zeros input word is mappedto a predetermined constellation point. Otherwise, control jumps to stepS150.

[0083] In step S150, the receiver sends the B_(i), G_(i) and X_(i)tables which contain the bit patterns of the chosen constellation pointsfor each carrier that map the all-zeros input word (w_(i)) to the Q-modeconstellation point (Q_(i)). Then, in step S160, based on the chosenconstellation point, both the transmitter and receiver generate the XORscrambler and phase rotation for each carrier. Control then continues tostep S170 where the control sequence ends.

[0084]FIG. 8 outlines an exemplary method of entering Q-mode accordingto this invention. In particular, control begins in step S200 andcontinues to step S210. In step S210, an all zero bit pattern isreceived by the transmitter. Next, in step S220 a determination is madewhether the Q-mode symbol will be sent to the receiver. If the Q-modesymbol is to be sent to the receiver, thus allowing the transmitter toenter Q-mode, control continues to step S250. Otherwise, control jumpsto step S230.

[0085] In step S250, the Q-mode symbol is forwarded to the receiver.Next, in step S260, the transmitter enters Q-mode. Then, in step S270,the receiver demodulates the Q-mode symbol and continues processing asif the receiver was operating during normal showtime. Control thencontinues to step S280 where the control sequence ends.

[0086] In step S230, the transmitter transmits the modulated all-zerosymbol. Then, in step S240, a determination is made whether themodulated all-zero symbol is the same as the Q-mode symbol. If themodulated all-zero symbol is the same as the Q-mode symbol controlcontinues to step S260 where the transmitter enters Q-mode. Otherwise,control continues to step S280 where the control sequence ends.

[0087]FIG. 9 outlines an exemplary method of exiting Q-mode according tothis invention. In particular, control begins in step S300 and continuesto step S310. In step S310, a determination is made whether actualinformation bits are to be transmitted. If actual information bits areto be transmitted, control continues to step S300. Otherwise, controlcontinues to step S320 where the transmitter remains Q-mode. Controlthen returns back to step S310.

[0088] In step S330, the transmitter will exit Q-mode by sending DMTsymbols modulated with the real information bits. Next, in step S340,the receiver will receive and seamless demodulate the transmitted actualinformation bits. Control then continues to step S350 where the controlsequence ends. Thus, the receiver will have transitioned through thetransmitter cycling into and out of Q-mode without the receiver needingto detect this transition nor detect the exit of the transmitter fromQ-mode.

[0089] The present invention for a receiver transparent Q-mode in amulticarrier transmission system can be implemented either on a DSLmodem, such as an ADSL modem, or separate programmed general purposecomputer having a communication device. The present method can also beimplemented in a special purpose computer, a programmed microprocessoror a microcontroller and peripheral integrated circuit element, an ASICor other integrated circuit, a digital signal processor, a hardwired orelectronic logic circuit such as a discrete element circuit, aprogrammable logic device, such as a PLD, PLA, FPGA, PAL, or the like,and associated communications equipment.

[0090] Furthermore, the disclosed method may be readily implemented insoftware using object or object-oriented software developmentenvironments that provide portable source code that can be used on avariety of computers, workstations, or modem hardware and/or softwareplatforms. Alternatively, the method may be implemented partially orfully in hardware using standard logic circuits or a VLSI design. Othersoftware or hardware can be used to implement the methods in accordancewith this invention depending on the speed and/or efficiencyrequirements of the system, the particular function, and the particularsoftware and/or hardware or microprocessor or microcomputer(s) beingutilized. Of course, the present method can also be readily implementedin hardware and/or software using any known later developed systems orstructures, devices and/or software by those of ordinary skill in theapplicable art from the functional description provided herein and witha general basic knowledge of the computer and telecommunications arts.

[0091] Moreover, the disclosed methods can be readily implemented assoftware executed on a programmed general purpose computer, a specialpurpose computer, a microprocessor and associated communicationsequipment, a modem, such as a DSL modem, or the like. In theseinstances, the methods and systems of this invention can be implementedas a program embedded in a modem, such as a DSL modem, or the like. Themethods can also be implemented by physically incorporating operationequivalents of the methods into software and/or hardware, such as ahardware and software system of a multicarrier information transceiver,such as an ADSL modem, VDSL modem, network interface card, or the like.

[0092] While this invention has been described in conjunction with anumber of embodiments, it is evident that many alternatives,modifications and variations would be or are apparent to those ofordinary skill in the applicable art. Accordingly, applicants intend toembrace all such alternatives, modifications, equivalents and variationsthat are within the spirit and the scope of this invention.

We claim:
 1. A multicarrier communications system transmittercomprising: an XOR scrambler; an encoder; a phase rotator; and aninverse fast fourier transform module, the module outputting a pluralityof symbols, at least one of the symbols being a Q-mode symbol thatindicates the transmitter is entering a Q-mode, wherein the Q-modesymbol allows the transmitter to seamlessly transition into and out ofthe Q-mode independent of an operational state of a receiver.
 2. Thesystem of claim 1, wherein the Q-mode symbol is based on a pseudo-randomquadrature amplitude modulated DMT symbol.
 3. The system of claim 1,wherein at least one of the transmitter and the receiver determine oneor more fine gain values and a number of bits modulated on each of aplurality of carriers.
 4. A method for a transmitter to enter a Q-modecomprising: receiving an all zeros bit pattern; transmitting a Q-modesymbol; and entering a Q-mode, wherein the entry and exit of the Q-modeis independent of an operational state of a receiver.
 5. The method ofclaim 4, further comprising determining one or more fine gain values anda number of bits modulated on each of a plurality of carriers.
 6. Themethod of claim 4, further comprising mapping an all zeros word to apredetermined constellation point.
 7. The method of claim 4, furthercomprising determining an XOR scrambler characteristic and a phaserotation based on a constellation point.
 8. A protocol for a transmitterto enter a Q-mode comprising: receiving an all zeros bit pattern;transmitting a Q-mode symbol; and entering a Q-mode, wherein the entryand exit of the Q-mode is independent of an operational state of areceiver.
 9. The protocol of claim 8, further comprising determining oneor more fine gain values and a number of bits modulated on each of aplurality of carriers.
 10. The protocol of claim 8, further comprisingmapping an all zeros word to a predetermined constellation point. 11.The protocol of claim 8, further comprising determining an XOR scramblercharacteristic and a phase rotation based on a constellation point. 12.A means for a transmitter to enter a Q-mode comprising: means forreceiving an all zeros bit pattern; means for transmitting a Q-modesymbol; and means for entering a Q-mode, wherein the entry and exit ofthe Q-mode is independent of an operational state of a receiver.
 13. Themeans of claim 12, further comprising means for determining one or morefine gain values and a number of bits modulated on each of a pluralityof carriers.
 14. The means of claim 12, further comprising means formapping an all zeros word to a predetermined constellation point. 15.The means of claim 12, further comprising means for determining an XORscrambler characteristic and a phase rotation based on a constellationpoint.
 16. An information storage media comprising information that fora transmitter to enter a Q-mode comprising: information that receives anall zeros bit pattern; information that transmits a Q-mode symbol; andinformation that allows the transmitter to enter a Q-mode, wherein theentry and exit of the Q-mode is independent of an operational state of areceiver.
 17. The media of claim 16, further comprising information thatdetermines one or more fine gain values and a number of bits modulatedon each of a plurality of carriers.
 18. The media of claim 16, furthercomprising information that maps an all zeros word to a predeterminedconstellation point.
 19. The media of claim 16, further comprisinginformation that determines an XOR scrambler characteristic and a phaserotation based on a constellation point.
 20. A method of conservingpower in a multicarrier communication system comprising: transmitting aQ-mode symbol indicating a transmitter is entering a Q-mode; seamlesslytransitioning into and out of the Q-mode, wherein a receiver demodulatesboth the Q-mode symbol and one or more information bits.